High speed on-off switching of laser diode arrays requires the fabrication of laser diode array assemblies with the lowest inductance possible. To accomplish lower inductance, the current path into the array (from the power source), through the array, and out of the array (back to the power source) needs to be as short as possible. The input and output lines through the array need to be as close to each other as possible and, if possible, parallel or substantially parallel to each other.
Laser diode arrays can be run continuously, in a manner known as continuous wave (CW) operation. CW operation usually involves providing drive current to the arrays on the order of 0 to 40 amps continuously.
Laser arrays can also be operated in a pulsed fashion, which is known as quasi continuous wave (QCW) operation. QCW operation usually involves providing drive current to the arrays on the order of 0 to 200 amps. Even higher drive currents are expected in the near future. Pulsed operation usually involves pulse widths in a range of less than 1 us to more than 100 ms, with most applications being in the 100 to 500 us pulse width range. These pulses are then usually repeated at intervals from 1Hz to several kHz.
FIG. 1 shows a configuration of a typical laser diode array 100 with laser diode bars 105 disposed in a substrate 115. When current is applied to the array 100, the laser diode bars 105 emit laser light generally in a direction 110.
To apply current to the array 100, a cathode connection 120 and an anode connection 130 are connected to opposite sides of the array 100. As also shown in FIG. 1, a heatsink 140 is provided below array 100, and insulation 150 is interposed between heatsink 140 and the respective cathode and anode connections 120, 130.
Metallization 155 around the substrate 115 and, as necessary or appropriate, around the laser diode bars 105 (except over the emission points of those bars, of course), enables the wrapping of current around the end of the laser diode array. The current flows in the direction of arrows 160.
In the just-described arrangement, distances between cathode and anode connections 120, 130 will be a function of the width of the heatsink 140, which in turn will be a function of the width of the laser diode array 100. The distance between those connections 120, 130 tends to be relatively substantial, on the order of 10 mm, depending on considerations such as the number of laser diode bars in the array 100 (dictating, to some extent, the width of the array 100); the dimensions of the heatsink 140 on which the array is mounted; the amount of insulation 150 that is needed between the cathode and anode connections 110, 120 and the heatsink 140; and the like.
With the configuration of FIG. 1, pulsed operation like that described earlier is attainable.
When operating at somewhat longer pulse widths, on the order of 100 us and above, the xe2x80x9cRise and Fall Timexe2x80x9d of the current pulse is of less of a concern. The xe2x80x9cRise and Fall Timexe2x80x9d of a current pulse from 0 current to full drive, say, 100 amps, and then from full drive current back to 0 amps is usually on the order of 1 us to 20 us, which is not a large concern when the actual pulse width itself is on the order of 100 us or more.
The xe2x80x9cRise and Fallxe2x80x9d time of a current pulse becomes more of a concern as the actual pulse width becomes shorter and shorter, and the intervals between pulses become shorter and shorter. For example: If the required pulse width is 1 us, then the rise and fall time of the current pulse has to be considerably shorter. Rise and fall times are affected greatly by the inductance of the current path, especially when trying to switch on and off high current pulses. So, in order to have faster rise times, it is important to keep the inductance of the entire system as low as possible.
Inductance in high current pulsed systems can be minimized by keeping the power leads as close to each other as possible and, if possible, as parallel or substantially parallel to each other as possible, in order to cancel out magnetic fields which high current flows create. However, while making power leads that are close and parallel to each other is relatively easy, creating the same lower inductance conditions in laser diode arrays is not.
Because of prior operating parameters for laser diode arrays, it has not been necessary to investigate the need for low inductance arrays. However, as needs have changed, with shorter pulse widths and faster repetition rates, it has become necessary to investigate array configurations, and in particular anode/cathode configurations, which will provide sufficiently low inductance to enable fast rise and fall times.
In view of the foregoing, there has arisen a need for laser diode arrays, still capable of operating at very high drive currents, on the order of 50 to 200 amps per pulse, but operating with pulse widths of less than 1 us and at very short intervals between pulses, leading to laser diode arrays with pulsed operations on the order of 100 kHz and above, more than two orders of magnitude higher than QCW rates at which laser diode arrays have typically operated. It is the need for these shorter and shorter pulse widths, combined with higher and higher drive currents, that has led to a need to fabricate of laser diode arrays with the lowest inductance possible.
To achieve these and other objects, according to the present invention, a low inductance laser diode array is provided in which, in an exemplary embodiment, the cathode and anode connections to the array are positioned substantially parallel to each other, and more closely together than has been the case previously.